Pressure sensor

ABSTRACT

A pressure sensor for measuring high pressures, the sensor being adapted to act externally on a measuring element, the measuring element having a central cavity and being composed of two parts which are sealingly joined in order to form the cavity, and comprising sensor means for measuring the mechanical stress condition of the measuring element. The two parts of the measuring element are manufactured in planar technology form silicium or quartz and have a substantially larger length than lateral dimensions, the cavity being oriented in a longitudinal direction, and the sensor means being piezo-resistive elements lying adjacent to the external or internal surface of the measuring element.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to a pressure sensor in particular for very highpressures, preferably adapted to act externally on a measuring element,the measuring element having a central cavity and being composed of twoparts which are sealingly joined in order to form the cavity, andcomprising sensor means for the mechanical stress condition of themeasuring element when subjected to pressure.

In principle a pressure sensor consists of a spring element (measuringelement) and a measurement or sensor device. Usual commerciallyavailable pressure sensors based on silicium technology can employmembranes as spring element, provided with piezo-resistive resistors assensor means. Spring elements as membranes are unfavorable at highpressures because they are sensitive to effects related to the clampingin a substrate with a transition to materials having an unequal modulusof elasticity. The stress detected in such membranes will be acombination of compressive and tensile stresses. If the tensile stressesbecome sufficiently high, a break can occur. At large deformations thestresses in a membrane will not be a linear function of pressure, whichresults in a non-linear signal.

Pressure sensors for high pressures as indicated above, are in generalpreviously known, and a specific example may be found in Norwegianpatent application No. 94.0785. A pressure sensor according to Norwegianpatent application No. 94.0785 is based upon the Bourdon effect, with acapacitive reading of the deflection. This known pressure sensor isbased on tubular measuring elements, but the internal cavity or cavitiesis/are always disposed eccentrically, so as to obtain a bending of theelement. When such bending takes place shear stresses can be generatedin the joints, which in turn can lead to failure therein. A drawbackwith capacitive reading is that when very high pressures are concerned,it is desireable to have very small outer diameters, which results inextremely small capacitances. It is very difficult to detect these.Another drawback with capacitive detection consists therein that themeasuring element can not be directly subjected to the process medium.This is because the capacity is sensitive to the dielectric constant ofthe medium and will be very sensitive to contamination in the form ofparticles which can get into the capacitor gap.

In the book "Instrumenteringsteknikk" by Ole A. Solheim, Tapirpublishers Trondheim 1966, there is a description on page 118 of themeasurement principle employed here. According to the book thismeasurement principle comprises the attachment of strain gauges on atubular measuring element. On the surface of the element the stresses inthe azimuthal direction will always be twice as high as in the axialdirection. The difference in stress between these two directions will beproportional to the pressure. If there is provided for application ofthe highest pressure externally of the tubular element, tensile stressesthat can lead to breaking are avoided. The known measurement principleis in part utilized in a more modern design according to European patentpublication 0.107.549. The designs according to this publication,however, do not take advantage of the difference between axial andradial stress upon external or internal pressure application, but areonly based on deformation changes. The sensor devices operate withacoustic surface waves, which means that the complete pressure sensormust have total dimensions above a certain lower limit.

Another and somewhat less interesting example from the patentliterature, is to be found in French patent publication 2.531.533.

SUMMARY OF THE INVENTION

On the background of the previously known techniques, a pressure sensoras recited in the introduction above, has novel and specific featuresconsisting in the first place therein that the two parts of themeasuring element are manufactured in planar technology, preferably ofsilicium or quartz, and have a substantially larger length than lateraldimensions, with the cavity oriented in the longitudinal direction, andthat the sensor devices are in the form of piezo-resistive resistiveelements lying adjacent to an external or an internal surface of themeasuring element.

One or both parts of the measuring element is/are provided with elongaterecesses or channels from one of the major surfaces thereof, so thatwhen the two parts are joined, the cavity mentioned will be formed, andin general the cavity can be regarded as tubular. Preferably themeasuring element according to the invention has a length being at leastof an order of magnitude ten times the lateral dimensions. Normally astraight design is preferred, so arranged that a difference in pressureat the inside and the outside can be detected. It is an advantage thatthe geometry of the measuring element serves to avoid bending (Bourdoneffect) when subjected to pressure, and that shear stresses in thejoining surfaces as well as tensile stresses are minimized. As will beseen from the following description this is favorably obtained by havingthe cavity located centrally in the cross section of the measuringelement. The cavity is either made available for a reference pressurefor differential pressure measurement, or is closed with a confinedreference pressure, preferably vacuum, for measuring absolute pressures.The outside of the measuring element can be designed for the purpose ofbeing subjected to the pressure to be measured, either by being directlysubjected to a process medium or to a known or selected medium beingseparated from the process medium by means of an isolating membrane in amanner known per se.

By having the invention based on the use of micromechanicalmanufacturing techniques, namely planar technology, it is possible toproduce measuring elements according to the invention with very smalldimensions, which is particularly advantageous when the measurement ofextremely high pressures is concerned. Moreover the elongate design isvery favorable in view of required mounting, integration or packaging ofthe actual measuring elements in practical measuring apparatuses.

It is an independent aspect per se of this invention, to provide a noveland specific measuring apparatus having associated supporting ormounting means in a surrounding housing or holding member.

BRIEF DESCRIPTION OF DRAWINGS

In the following description the invention will be explained moreclosely with reference to various embodiments being illustrated asexamples on the drawings, wherein:

FIG. 1 in perspective view shows a first embodiment of a measuringelement according to the invention, mounted in a holder,

FIG. 2A shows an enlarged cross section of the main part of themeasuring element in FIG. 1,

FIG. 2B in enlarged cross section shows an abutment portion of themeasuring element in FIG. 1, close to the holder mentioned above,

FIG. 3 in corresponding cross section as in FIGS. 2A and 2B, shows avariant of the cross-sectional shape of the measuring element, i.e. thecross section of the cavity,

FIG. 4 shows another variant of the cross-sectional shape, i.e. arectangular main shape,

FIG. 5 in simplified elevation view shows a measuring apparatus with ameasuring element according to the invention incorporated therein,

FIG. 6 shows the apparatus of FIG. 5 from the (left-hand) end,

FIG. 7 in elevation shows a specific embodiment of a measuring elementaccording to the invention, with a cavity subdivided into individualchannels,

FIG. 8 shows the measuring element of FIG. 7 from above,

FIG. 9 shows the measuring element in cross-sectional view along theline IX--IX in FIG. 7, and

FIG. 9A is an enlarged detail of the cross section in FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the example shown in FIG. 1 an elongate measuring element 1 iscomposed of two parts 1A and 1B. These are identical and form togetheran internal cavity 3 between two end walls 3A and 3B. Thus the cavity 3does not extend quite to the ends of the main part of the measuringelement 1, which is here shown with an octogonal cross section.

A surface 5 of the measuring element is provided with sensor devices11,12,13 and 14, which through a number of leads 15 are connected toexternal electronics or measuring circuits. Sensor means 11,12,13 and 14are in the form of piezo-resistive elements being known per se and beingpreferably arranged in a bridge circuit. The sensor is sensitive tochanges in the mechanical stress condition or deformation of themeasuring element when subjected to varying pressure differences betweenthe ambient medium and the internal cavity 3.

At the inner end thereof the octogonal main part 1 of the measuringelement continues as an abutment portion 2 having a square outercross-sectional contour, for the purpose of cooperating with a holdingmember 18 in which the whole measuring element is mounted. Thus holdingmember 18 has a through hole 19, being preferably circular, and adaptedto receive a mounting portion 7 of the measuring element, this portion 7having advantageously the same outer cross sectional contour as the mainpart 1. Abutment portion 2 serves to provide for a secure engagement ofthe measuring element against holding member 18, this being ofparticular interest when the measuring element is subjected to very highexternal pressures. Such pressures will seek to force the measuringelement 1 in axial direction into the hole 19, but the projectingcorners, 2A,2B,2C and 2D (see also FIG. 2A), will effectively blockagainst displacement of the measuring element under such pressureapplication. Through the hole 19 the mounting portion 7 can beadditionally fixed by means of a suitable adhesive which fills theinterspace between the octogonal cross section and the circular hole 19.

FIG. 2A in more detail shows the cross-sectional shape of the measuringelement, comprising the two parts 1A and 1B which are joined at theplane being indicated at 8. In FIG. 2A the four corners 2A,2B,2C and 2Dare shown, serving for abutment or engagement against holder 18.

The square outer cross-sectional contour in FIG. 2B corresponds to thecross section of abutment portion 2, but the inner cavity 3 actually isnot present within abutment portion 2 when absolute pressure measurementis concerned, i.e. with a closed cavity 3 between end walls 3A and 3B asin FIG. 1. Thus FIG. 2B can be considered to illustrate an alternativemain shape of the effective length of a measuring element 1, where thecavity 3 can have the same cross sectional shape as in FIG. 2A andFIG. 1. Thus according to the invention it is an advantage to design thecavity 3 with a diamond shaped cross section, having two S cornerspositioned in the joining plane 8 between the two parts 1A and 1B.

FIG. 3 shows a variant whereby the outer cross-sectional contour ofmeasuring element 31 is the same as in FIG. 2A, i.e. octagonal, whereasthe inner cavity 33 has a hexagonal cross-sectional profile. This insimilarity to cavities 3, having a diamond shape, is well suited formanufacturing in planar technology, in particular by etching. Inaddition to a rectangular and octogonal outer cross-sectional contour,the measuring element according to the invention can be manufacturedwith a polygonal outer cross-sectional contour in other variants, suchas a hexagonal cross section.

Based upon planar technology as mentioned, the two parts of themeasuring element can be advantageously joined by means of anodicbonding or also by so-called fusion bonding. Whereas the measuringelement cross sections being illustrated as examples in FIGS. 1,2A,2Band 3 are symmetrical about the joining plane 8 (and besides about acentral plane normal thereto), the rectangular cross section in FIG. 4is composed of two parts 21A and 21B which do not have the samecross-sectional shape. Part 21A is thicker and is the only one providedwith a recess 23 forming the desired cavity. A joining plane is shown at28, whereby part 21B quite simply is a suitable plate-shaped partforming together with part 21A the complete cross section of measuringelement 21. Also in this embodiment, however, the cavity 23 will belocated centrally and symmetrically within the total cross section ofmeasuring element 21.

FIGS. 5 and 6 show a practical example of a more complete measuringapparatus incorporating a measuring element according to the invention.A housing or a holder as shown in FIG. 5 comprises three main parts,i.e. a sleeve part 41 having a central bore 41A, an inner part 42 whichpreferably can be provided with external threads, and an intermediatepart 43 being externally designed as a nut head. Sleeve part 41 isintended to be inserted into the zone where a pressure is to bemeasured, since the effective main part of a measuring element 51 ismounted so as to project out into bore 41A. At the other side the innerpart 42 is adapted to be located at the low pressure side, provided thatthere is a higher pressure to be measured by means of measuring element51. Intermediate part 43 can be considered to comprise a holding member48 corresponding to holding member 18 in FIG. 1., with a hole 49 havinga wider portion 49A for receiving the abutment portion of the measuringelement corresponding to portion 2 in FIG. 1. A mounting portion 57belonging to measuring element 51, penetrates through hole 49 andfurther into the interior of part 42, for electrical connection toelectronics therein or electrical circuits being located outside at thelow pressure side.

Finally FIGS. 7,8,9 and 9A show a particular embodiment according to theinvention, whereby the measuring element has a strongly flattenedcross-sectional shape, as will be seen specifically from FIGS. 9 and 9A.Two parts 61A and 61D manufactured in planar technology are here as inthe previously described embodiments, joined so that the measuringelement 61 will have a typical plate shape. Here the internal cavity issubdivided into a large number of separate channels, of which fourchannels are shown at 63 in FIG. 9A. Such an arrangement of severalparallel and longitudinal cavities or channels in measuring element 61,in certain respects can be advantageous. The effect as regards detectionof stresses occurring when the measuring element is subjected topressure, also here can be detected favourably by means of sensordevices in the form of piezo-resistive elements on or adjecent to amajor surface, for example the surface 65 as shown in FIG. 7. Sensordevices with electrical leads are not particularly shown in any of FIGS.7-9A, but can be arranged in a quite corresponding manner as shown atsurface 5 in FIG. 1.

At 60 in FIG. 7 there is schematically indicated a boundary between thehigh pressure side and the low pressure side, a mounting member 68 beingprovided at the major surface 65 of the measuring element, for exampleby anodic bonding. Mounting member 68 is hollow as shown at 69, forcarrying electrical leads out from the sensor devices (not shown) onmeasuring element 61. The hollow interior 69 of mounting member 68 thusis adapted to communicate with the low pressure side, so that apractical and advantageous transition between the high pressure side andthe low pressure side is obtained, among other things in view of therequired electrical connection from the sensor devices. As is also thecase in FIG. 1, the piezo-resistive elements here will be advantageouslyintegrated into the structure of the measuring element, i.e. inparticular part 61B adjacent to surface 65. The piezo-resistive elementas well as leads thereto more specifically can be arranged as buriedinside the surface concerned at the measuring element, i.e. the surface5 in FIG. 1 and surface 65 in FIG. 7.

The form of mounting or "packaging" of measuring element 61 as appearsfrom FIGS. 7 and 8, whereby mounting member 68 is a substantialcomponent, is advantageous in certain cases, and this has substantialsignificance in a complete measuring apparatus. The manner of mountingin this embodiment is closely related to the strongly flattened,plate-like shape of measuring element 61.

As already explained above the mounting or packaging of the more"normal" measuring element 1 in FIG. 1, is rather different from thepackaging method in FIGS. 7 and 8. Besides it will be realised that thepackaging method according to FIG. 1 (and FIG. 5) can be of interest assuch also for measuring elements of other design and for other purposesthan what is described here. In all such variants, however, it will bean advantage that the measuring element is much elongated, as discussedabove, such as with a length dimension typically being of an order ofmagnitude as ten times the lateral dimensions of the element. Such anelongate shape is very practical for this particular packaging method,although for the production economy per se it could have been attractiveto employ relatively shorter elements. Of substantial significance bothwith respect to production and for the actual mounting, is the abutmentportion 2 described in this embodiment. An important detail in thisconnection is that the abutment faces at the projecting corners 2A,2B,2Cand 2D (see also FIG. 2A) can extend at an inclination and not bedirectly radial in a plane at right angle to the axis of the measuringelement.

In connection with mounting, packaging and encapsulation of such ameasuring element, it is obvious that on the surface thereof aprotective film can be applied, for example of Si₃ N₄, or apolyamid-plastic material, so that the measuring element can be directlyexposed to the pressure medium concerned.

Also a number of other modifications and variants are possible withinthe framework of the invention, perhaps in particular related to thedesire of obtaining efficient and economic production processes. Forexample each of the two parts of the measuring element per se can be anassembled structure, where layers of different types of materials can beincluded. Moreover it is obvious that for pure strength reasons acircular internal cavity would have been the ideal shape, but withrespect to production such a shape is not advantageous. A still further,possible modification consists therein that sensor devices can beprovided at more than one surface of the measuring element, for exampleat two major surfaces facing oppositely from each other.

What is claimed is:
 1. A pressure sensor for high pressures comprising ameasuring element having an external surface and a central cavitydefined by an internal surface of said measuring element and beingcomposed of two parts which are sealingly joined in order to form thecavity, and comprising sensor means for measuring a mechanical stresscondition of the measuring element wherein:the two parts of themeasuring element are manufactured in planar technology and have asubstantially larger length than lateral dimensions, the cavity beingcentrally located within said measuring element, oriented in thelongitudinal direction, and having a cross-sectional shape that issymmetric about at least two longitudinal planes, and said sensor meansbeing piezo-resistive elements lying adjacent to at least one of saidexternal and said internal surfaces of the measuring element.
 2. Apressure sensor according to claim 1, wherein the length of themeasuring element is at least of ten times the lateral dimensions.
 3. Apressure sensor according to claim 1 wherein the outer cross sectionalshape of the measuring element is selected from rectangular, hexagonaland octagonal shapes and symmetric about two longitudinal planes.
 4. Apressure sensor according to claim 1 wherein the cross section of thecavity is diamond shaped with two opposite corners located in thejoining plane between the two parts forming the measuring element.
 5. Apressure sensor according to claim 1 wherein the two parts haveidentical cross sectional shapes.
 6. A pressure sensor according toclaim 1 wherein said two parts have different thicknesses and that thecavity is formed by a recess in both of the parts, from the surfacefacing the other part.
 7. A pressure sensor according to claim 1 whereinthe two parts are joined by anodic bonding.
 8. A pressure sensoraccording to claim 1 wherein the two parts are joined by fusion bonding.9. A pressure sensor according to claim 1 wherein the cavity is adaptedto receive a reference pressure.
 10. A pressure sensor according toclaim 1 wherein the measuring element is mounted in a holder having acircular hole defined through a wall of said holder and forming ameasuring element-mounting portion of said holder, wherein the measuringelement is disposed adjacent to said mounting portion in the hole at ahigh pressure side of said wall and wherein said measuring element isprovided with an abutment portion the outer cross-sectional contour ofwhich at certain places is located radially outside the circumference ofthe hole.
 11. A pressure sensor according to claim 10, wherein the outercross-sectional contour of the mounting portion comprises radiallyprojecting comers in relation to the otherwise polygonal outercross-sectional shape of the measuring element.
 12. A pressure sensoraccording to claim 10 wherein leads to the piezo-resistive elements areintegrated into the structure of the measuring element.
 13. A pressuresensor according to claim 12 wherein the piezo-resistive elements andleads thereto, are buried within the surface of the measuring element.14. A pressure sensor according to claim 1 wherein the piezo-resistiveelements are arranged in an electrical bridge circuit.
 15. A pressuresensor according to claim 1 wherein said measuring element comprises aflattened cross-section so as to define a major external surface andsaid cavity comprises a plurality of individual channels arranged sideby side.
 16. A pressure sensor according to claim 15, wherein said majorsurface of the measuring element is joined to a hollow mounting member,said hollow mounting member being externally adapted to be subjected tohigh pressure and internally adapted to be located at low pressure andfurther wherein leads for said sensor means extend outwardly from saidsensor means into the hollow mounting member.
 17. A measuring apparatusfor measuring high pressures comprising:a measuring element having aside that is externally subjected to the high pressure and thereby to arelatively large force, wherein the measuring element is manufactured inplanar technology and is mounted in a housing having a through-holedefined in a wall of said housing and forming a measuringelement-mounting portion of said housing, the measuring element beingprovided with an abutment portion adjacent to said measuringelement-mounting portion at said high pressure side of said measuringelement, and wherein the outer cross sectional contour of said measuringelement at certain places is located radially outside the circumferenceof the hole.
 18. A measuring apparatus according to claim 17, whereinthe outer cross-sectional contour of the abutment portion comprisesradially projecting corners in relation to a polygonal outercross-sectional shape of the measuring element.
 19. A pressure sensoraccording to claim 1 wherein high pressure acts externally upon saidmeasuring element.
 20. A pressure sensor according to claim 1 formed ofsilicium.
 21. A pressure sensor according to claim 1 formed of quartz.22. A pressure sensor according to claim 11 wherein said mountingportion comprises a polygonal outer cross-sectional shape.
 23. Ameasuring apparatus according to claim 17 formed of silicium.
 24. Ameasuring apparatus according to claim 17 formed of quartz.
 25. Ameasuring apparatus according to claim 18 wherein said mounting portioncomprises a polygonal outer cross-sectional shape.